Process for preparing kraft pulping liquor from black liquor including separate carbonation with combustion gases and evaporation steps



United States Patent Ofice 3,525,666 Patented Aug. 25, 1970 US. Cl.162-30 Claims ABSTRACT OF THE DISCLOSURE A process is provided forrecovering economically sulfur lost as hydrogen sulfide from blackliquor during evaporation in black liquor reclaiming processes, andreusing the sulfur content of the hydrogen sulfide in preparing whiteliquor for pulping in the kraft process. The hydrogen sulfide isobtained from the black liquor by carbonating the black liquor to a pHbelow 11 with combustion gases containing at least 15% CO and thereafterstripping hydrogen sulfide from the resulting liquor by evaporating theliquor without addition of combustion gases, oxidizing the hydrogensulfide to sulfur, and dis solving the sulfur in a sulfide-containingaqueous solution to form fresh white liquor.

This application is a continuation-in-part of Ser. No. 482,908, filedAug. 26, 1965, now abandoned.

The preparation of chemical and semichemical cellulose pulps from woodrequires considerable quantities of cooking chemicals. Rydholm, PulpingProcesses, Interscience Publishers, New York (1965), Chapter 11, page764, indicates that from 200 to 500 kg. per ton of pulp are used.Therefore, as Rydholm indicates, it is of vital economic interest torecover as much of the spent chemicals as possible from spent blackliquor, and use them in the preparation of fresh white cooking liquor.Accordingly, a number of recovery processes have been developed, and arein use in the U.S.A., Sweden, and elsewhere.

The black liquor or waste liquor from the kraft pulp cooking operationscontains the cooking chemicals in a dilute solution, which inmostinstances has to be concentrated before the chemicals can berecovered and regenerated. In order to dispose of the organic substanceswhich are contained in the waste liquor, most recovery processes requirethat they be burnt, using the heat liberated in the combustion, forexample, in evaporation of black liquor. The inorganic chemicals canthen be recovered from the combustion residue or smelt, and used in thepreparation of a fresh batch of cooking or white liquor.

At pages 766 and 767, Rydholm summarizes the steps in ten chemicalsrecovery processes for use in various pulp cooking processes. Theseprocedures generally include the steps of evaporation of the liquor,combustion of the organic materials, recovery of chemicals from theresidue, and the conversion of these into a form which can be used inthe preparation of fresh cooking liquor.

In a pulping mill based on the kraft process, the principal inorganicchemicals recovered as the result of the recovery process are sodiumcarbonate and sodium sulfide. Since the cooking liquor consists mainlyof hydroxide and sulfide, causticizing is necessary to convert thecarbonate to hydroxide. The kraft cooking pulping liquor also containslignins and organic acids in the form of their sodium salts, as well assodium hydrosulfide. The organic materials are destroyed by combustion.A typical kraft black liquor, according to Rydholm, page 774, has thefollowing composition:

BLACK LIQUOR (TOTAL SOLIDS 17-22 PERCENT) Percent of total solids Alkalilignin 41 Extractives 3 Hydroxy acids and lactones 28 Acetic acid 5Formic acid 3 Methanol 1 Sulfur 3 Sodium 16 Total 100 This liquor isevaporated, and sodium sulfate is added prior to combustion as make-upto replace losses of sodium and sulfur. There are losses of sulfurduring the evaporation, as hydrogen sulfide, and there are also lossesof sulfur during the combustion, as sodium sulfate and S0 and theresidue from the combustion, as indicated, is composed of sodiumcarbonate and sodium sulfide. Thus, in the usual recovery process theonly sulfur recovered from the waste kraft liquor is in the form ofsodium sulfide, as a residue after the combustion.

Although it is recognized that during the evaporation a considerableamount of sulfur is lost as hydrogen sulfide, it has been impractical torecover this hydrogen sulfide in the usual recovery process. The totalsolids content of a normal black liquor is about 20 percent, but it canbe considerably less, as low as 15 percent, depending on the moisturecontent of the wood, and the degree of dilution in the washingoperation, and to a lesser extent, the amount of liquor charged to thecook, and the pulp yield. In order to obtain proper ignition and burningof the organic materials in the combustion step, it is necessary toconcentrate the liquor to a solids content of from 50 to percent. Thus,considerable amounts of water must be removed, and the amount isgenerally of the order of from 6 to 8 cubic meters per ton of pulp. Theliquor is quite alkaline, having a pH of 12 or more, and thus theconcentration of hydrogen sulfide in the escaping gases is rather low,so low, that it is impractical to separate it from the condensiblegases.

There are various techniques for evaporation of waste pulping liquors.Rydholm indicates that the two principal categories of evaporators arethe film type (such as a cascade evaporator) and the forced-circulationtype (such as a cyclone evaporator or a venturi scrubber). Kibrick,Scopp and Colton, US. Pat. No. 2,944,928, dated July 12, 1960, describethe use in addition of direct heat evaporators.

In this procedure (Britt, Handbook of Pulp and Paper Technology,Reinhold Publishing Corp. New York, 1964, page 189), the black liquor ispassed through multiple effect evaporators to an average of 45 to 5 5percent solids (concentrated from the original 15 to 20 percent solids),and is then further concentrated to between 62 and 70 percent solids ina direct-contact evaporator. The liquor and flue gases from the feedwater heater or air heater are brought into intimate physical contact.When this is done, however, rather large volumes of flue gases areneeded, and the flue gases further dilute the gases liberated from theevaporating liquor. Thus, the concentration of hydrogen sulfide in theresulting gases becomes even less than normal. Since the flue gases arenoncondensible, this greatly increases the difficulty of recovery ofhydrogen sulfide. Moreover, the flue gases include a high proportion ofoxygen, and the sulfide of the black liquor is readily oxidized tothiosulfate, corresponding to a molar ratio O :H S of 1:1. Suchreactions further reduce the concentration of hydrogen sulfide. Thus, inthe Kibrick process the recovery of hydrogen sulfide is even lesspractical than in the normal evaporation procedure.

Kibrick et al. indicate that the hydrogen sulfide is partially removedin the evaporators (presumably, during concentration from 15 to 20percent solids to 45 to 55 percent solids) as a result of the followingreaction:

They state, further, that in the direct heat evaporators (whereconcentration is increased to 62 to 70 percent) hydrogen sulfide isreadily evolved, but not completely in most reactions, as a result ofthe carbonation reaction which occurs as a result of the direct contactwith the flue gases. However, Kibrick et al. do not explain the natureof the carbonation reaction to which they refer, so it is not clear whatthey had in mind, nor do they give the carbon dioxide content of theflue gases they contemplate, or the pH of the liquor being evaporated,nor do they suggest how to recover hydrogen sulfide liberated in thisway from either the efiluent from the mulitple effect evaporators or thedirect contact evaporators.

The reasons why it is difiicult to recover hydrogen sulfide from thegases emitted from the evaporating liquor are readily seen by a study oftypical plant conditions. In a kraftpulp mill producing 20 tons per hourof pulp, yielding a black liquor containing 15 percent solids, anevaporation of the black liquor to 60 percent solids content requiresthe evaporation of 80 cubic meters of water per ton of pulp. This meansthat 60 kg. of water must be evaporated per cubic meter of liquor. Thisrequires a quantity of heat equivalent to 32,000 kilo calories per cubicmeter of liquor. The heat recovered from hot flue gases is about 60kilogram-calories per normal cubic meter, and thus to supply solely fromdirect contact with flue gases, as in the Kibrick et al. process, theamount of heat required to evaporate this much Water, 550 normal cubicmeters of flue gas are required, per cubic meter of unevaporated liquor.This is a very large volume of gas, which of course dilutes thenoncondensible gases liberated from the black liquor. Even if the fluegases are restricted to the direct contact evaporators, the volume isover 180 normal cubic meters.

If the available quantity of hydrogen sulfide liberated from the blackliquor in the course of the evaporation is 280 cubic meters per hour, itis seen that with 90,000 normal cubic meters per hour of the gases, thehydrogen sulfide concentration in the gases from the black liquor willbe only 0.3 percent. It is no greater in the effiuent from the directcontact evaporators. Such a small volume cannot be economicallyrecovered. Moreover, if the oxygen concentration in the flue gases is0.3 percent, there is a considerable probability that the main part ofthe hydrogen sulfide to be stripped off by the flue gases will beoxidized to thiosulfate, since one mole of oxygen is capable ofconverting one mole of sulfide to thiosulfate.

Despite the low concentrations of hydrogen sulfide that are liberated inthe normal evaporation process, it is apparent that the increasing costof the chemicals consumed in pulping requires economic recovery of asmany chemicals as possible, and this includes the sulfur content of theliquor that is lost during the evaporation stage. Moreover, heat economyrequires that this be done with the least possible expenditure of heat,consistent with concentration of the liquor to the desired solventscontent. It is thus most undesirable to dilute the gases from theevaporating liquor to the extent required in the process described inKibrick et al. No. 2,944,928.

Recovery of hydrogen sulfide from the waste gases is also important foranother reason. There is an increasing interest by local authorities inpreventing atmospheric pollution, and their concern is particularlydirected to preventing the liberation of noxious, foul-smelling gasesinto the atmosphere. This problem is particularly acute in regions nearpulping mills. Consequently, and process improvement which can result ina minimization or elimination of the liberation of such gases during theevaporation of waste pulping liquor becomes quite important.

In accordance with the instant invention, a process is provided for theeconomical recovery of hydrogen sulfide from the waste gases obtainedduring the evaporation of black liquor, and particularlypolysulfide-containing black liquor, obtained at the conclusion of thekraft pulping process. The process includes a preliminary preparation ofthe liquor for evaporation in a manner which ensures the liberation of arelatively high concentration of hydrogen sulfide in the gases (comparedto that normally liberated) and a high ratio of hydrogen sulfide tooxygen in such gases, while at the same time holding to a minimum thevolume of flue or combustion gases or other noncondensible gases broughtin contact with the evaporating liquor.

In the process of the invention, the black liquor from a kraft pulpingprocess is first carbonated with combustion gases containing at leastabout 15 percent carbon dioxide, in an amount to reduce the alkalineblack liquor to a pH below 11, the combustion gases being less thanabout 11 normal cubic meters per cubiF meter of available hydrogensulfide, and then hydrogen sulfide is stripped from the resultingliquor, while evaporating the liquor to increase the solids content toat least 45 percent, thereby removing a portion of the sulfur of theliquor as hydrogen sulfide, and forming a proportion of at least percenthydrogen sulfide in the noncondensible gases liberated from the blackliquor, recovering hydrogen sulfide from the gases evolved in theevaporation, oxidizing the recovered hydrogen sulfide to sulfur with airor sulfur dioxide, and dissolving the sulfur in a sulfide-containingaqueous solution to form a sulfide cooking liquor, and preferably apolysulfide cooking liquor. The sulfide-containing aqueous solution ispreferably obtained by combustion of the evaporated liquor, to form aresidue comprising a alkali carbonate and alkali sulfide, causticizingthe residue to form alkali hydroxide and alkali sulfide, and dissolvingthe residue in water.

An important feature of the applicants invention is the preliminarycarbonation of the black liquor with combustion gases that contain ahigh proportion of carbon dioxide, prior to the evaporation step. It isessential in order to obtain a high concentration of hydrogen sulfide inthe gases liberated from the black liquor that the dilutingnoncondensible gases be kept to a minimum volume. This can only be doneif the carbon dioxide content is high, and the carbonation step isseparate from the evaporation. The use of a high proportion of carbondioxide ensures that an effective carbonation is obtained, accompaniedby a sufficient reduction in pH to ensure that the following reactionsoccur:

(1) Na S+CO +H 0- NaHCO +NaHS (2) NaHS +CO +H O =NaHCO +H S (dissolved)Then, during stripping (evaporation), these reactions occur:

(3 NaHS +NaHCO Na CO +H ST (4) H S (dissolved) H ST If reactions (1) and(2) are not carried out first, as a preliminary step, the reaction thatoccurs is possibly that set out by Kibrick et al., above, or, moreprobably:

There is no upper limit on the proportion of carbon dioxide in thecombustion gases, but the normal combustion process, the carbon dioxidecontent of the combustion gases does not exceed about 21 percent, and isfrequently only within the range from 15 to 18 percent.

If the carbon dioxide concentration is high, then it is possible in thecarbonation step to use an amount of combustion gases that is less than20 normal cubic meters per cubic meter of unevaporated black liquor. Nocombustion gases are added during evaporation. This is to be contrastedwith the 550 normal cubic meters of flue gas per cubic meter ofunevaporated liquor required in the Kibrick process, as indicated above.

The pH of the black liquor is reduced to not over 11 during thecarbonation step. When alkalinity is reduced to below 11 by carbonation,the organic components present in the black liquor are not precipitated,provided the pH is not brought below about 9 and accordingly there areno complications in this decrease in pH. Preferably, the pH is broughtto from about 11 to about 9, by carbonation.

The carbonation makes it possible to increase the proportion of hydrogensulfide in the noncondensible gases liberated from the black liquorduring evaporation. The correlation between sulfur liberated as hydrogensulfide and pH is shown in the following table, which represents theamount of hydrogen sulfide recovered during the stripping operation,based on the total amount of sulfur in the liquor:

Percent S pH: lost as H 5 11.1 9

Thus, if more than 18 percent of the sulfur is to be liberated ashydrogen sulfide, a pH of from to 9 is used, and if less than 18 percentof the sulfur is to be liberated, as hydrogen sulfide, a pH of from 9 to11 is used.

The proportion can be adjusted as desired, since the sulfur remaining inthe liquor is recovered later, as sodium sulfide or sodium hydrosulfide(except for losses as S0 and sodium sulfate during combustion). Thus,the proportion can be set to give whatever ratio of sulfur to sodiumsulfide or hydrosulfide in the regenerated pulping liquor may bedesired. If a polysulfide pulping liquor is desired, a higher ratio ofhydrogen sulfide may be preferred.

It is desirable to liberate a larger-than-normal proportion of hydrogensulfide during the evaporation, so as to maintain an at least 90 percentconcentration of hydrogen sulfide in the noncondensible gases liberatedfrom the liquor, since this high concentration facilitates theconversion of the hydrogen sulfide to sulfur via the Clans process,without further purification. It may be more economical, from thestandpoint of sulfur recovery, to liberate a larger proportion ofhydrogen sulfide in the evaporation stage, and recover this as sulfur,leaving less to be recovered later as sodium sulfide or sodiumhydrosulfide, and reducing combustion losses as S0 and sulfate, than toattempt to retain all the sulfur in the liquor as sodium sulfide orsodium hydrosulfide, as was intended in prior procedures. In any case,it is well recognized that it is not possible to remove all of thesulfur from the liquor as hydrogen sulfide (and this, of course, isevidenced in the above), unless the pH is brought to the acid side,i.e., below 7, which is not desirable, because of precipitationproblems. Accordingly, in accordance with the invention, the pH of thewaste liquor is brought to within the range from about 8 to about 11, bycarbonation.

In the process of the invention, the carbonation is carried out as aseparate step prior to evaporation. In this Way, the combustion gasesemployed from the carbonation are not mixed with the gases liberatedfrom the liquor during evaporation. This makes it possible further toincrease the concentration of hydrogen sulfide in such gases.

By combustion gases is meant the gases that are obtained during thecombustion stage in the kraft black liquor recovery process. This stagefollows evaporation of the liquor. The organic constituents which arepresent after evaporation are simply burned, using conventionalprocedures, as described in Rydholm, Chapter 11, pages 776 to 791, thedisclosure of which is hereby incorporated by reference. The combustionis controlled in such a manner that the concentration of carbon dioxidein such combustion gases is at least 15 percent.

Following the carbonation step, the black liquor is then evaporated,from the original dilute condition to a solids concentration of at least45 percent, and preferably from 50 to 65 percent. This evaporation canbe carried out in one or in several stages, at atmospheric or reducedpressure, but for economic reasons it is usually carried out undervacuum, in several stages. The hydrogen sulfide is a component of thenoncondensible gases (the condensible gases being primarily steam), andthe noncondensible gases from each stage are collected, say, in a commonconduit, which is connected usually over a heat exchanger to a vacuumpump, ejector, or alkali wash. The heat content of the noncondensiblegases can be used in the heat exchanger to heat up waste liquor to beevaporated, thus conserving heat. The noncondensible gases can be passedto a vacuum pump, ejector or alkali wash.

Evaporation of a carbonated black liquor having a pH below 11 inaccordance with the invention will produce noncondensible gases of whichat least 90 percent by volume is composed of hydrogen sulfide. Air leaksin the recovery system may reduce this somewhat, and therefore, aboutpercent H S by volume is a practicable proportion in a commercial systemwith small amounts of air leaking into the system. Since the separationof condensible gases, such as steam, from the gases liberated from theevaporating liquor is rather easy, this means that in the process of theinvention, an efficient and effective recovery of hydrogen sulfide ispossible.

After separation from the condensible gases, the hydrogen sulfide canthen be recovered for oxidation by any of several techniques.

In one process, the hydrogen sulfide is removed from the evaporationsystem by connecting the common exhaust gas conduit to a pressureincreasing means, such as a centrifugal pump, a piston compressor, or ablower, increasing the pressure of the hydrogen sulfide to above thepressure prevailing in the evaporation system, so that the gas can thenbe brought to the oxidizing system.

In another embodiment, the hydrogen sulfide can be absorbed from thenoncondensible gases by passing them through or over a suitableabsorbent, such as for instance a solution of monoethanolamine,diethanolamine, triethanolamine, potassium bicarbonate or sodiumbicarbonate, or an ammoniacal solution of ammonium sulfate, desirablyunder vacuum and at a low temperature. After the absorbent is saturatedwith hydrogen sulfide, or nearly so, it can be transferred to a suitabledesorption apparatus, operating under a pressure slightly aboveatmospheric and at an elevated temperature, whereby hydrogen sulfide canbe stripped off, and then flowed directly to the oxidizing system, whilethe absorbent is returned to the absorption apparatus.

The hydrogen sulfide thus recovered is oxidized partly with air orsulfur dioxide in the Claus process by preheating a suitably controlledhydrogen-sulfide reaction mixture, and introducing it into a reactionbed, whereupon the resulting sulfur is condensed and separated.

The sulphur that is recovered can be dissolved directly in asulfide-containing aqueous solution to form polysulfide. Thesulfide-containing aqueous solution is preferably alkaline, and can beprepared either by dissolving the melt formed by combustion of blackliquor in water, after causticizing, or by dissolving separately, andthen blending the solutions, when ready for use, of sodium hydroxide,and sodium hydrosulfide or sodium sulfide. If a higher purity isrequired, the hydrogen sulfide derived from the black liquor evaporationcan be absorbed in alkaline aqueous solution, and this can be used asthe solution for dissolving sulfur.

The process can also be combined with the evaporation of black liquorfrom normal sulfate pulping, so that hydrogen sulfide is takentherefrom, and converted into elemental sulfur, while the resultingsulfur loss is compensated by liquor of higher sulfidity from thepolysulfide system, such as evaporated black liquor or dissolved melt.

The process of the invention is applicable to black liquor obtained fromthe usual kraft process. It is particularly applicable to black liquorobtained from a polysulfide kraft process. Polysulfide cooking isdescribed in Rydholm under Chapter 9, Section I, page 642. Polysulfidecooking gives an increase in yield, as compared to a normal kraft cook,and in general employs at least from 30 to 40 kg. per ton of wood ofpolysulfide sulfur for a substantial yield improvement. Sulfur is addedto the white liquor in order to increase the polysulfide sulfur content,but this is not normally done Well in advance of the cooking, since thepolysulfides may be destroyed at high alkalinities. Consequently, inthis procedure the best method is to separate the sulfide and carbonateof the recovered alkali smelt after the combustion process by fractionalcrystallization, causticize only the carbonate, and dissolve the sulfurin the sulfide solution, where the alkalinity is not excessive, andwhere the polysulfides formed are fairly stable.

The process of the invention is particularly useful in polysulfide kraftcooking, because the hydrogen sulfide is recovered in the form ofsulfur, which can be returned directly to the regenerated cooking liquorby dissolution in the sulfide solution obtained after the combustionstep in the recovery procedure. Further information on polysulfidecooking will be found in the Rydholm text, the disclosure of which ishereby incorporated by reference.

The following example in the opinion of the inventor represents apreferred embodiment of this invention.

EXAMPLE The process of the invention was applied in a kraft pulp mill,which produced 20 tons per hour of sulfate pulp, digested by means of aliquor containing 35 g. of poly sulfide sulfur per ton of pulp. Thetotal sulfur charged was 100 kg. per ton of pulp. The alkali losses inthe system corresponded to 8 kg. of sulfur per ton of pulp.

The process of the invention was applied to this mill as follows. The pHof the black liquor recovered from the polysulfide pulping was reducedto 9.0 by treatment with 3000 cubic meters per hour of combustion gasescontaining percent carbon dioxide. The carbonated black liquor was then.subjected to evaporation under vacuum. During evaporation, kg. of sulfurper ton of pulp was volatilized as hydrogen sulfide, corresponding to280 normal cubic meters per hour of hydrogen sulfide.

The gases from the evaporating liquor was passed through a watercondenser, so as to remove the condensible steam as water. The resultingnoncondensible gases including the hydrogen sulfide were at a pressureof 0.15 atmosphere, and a temperature of 60 C. Over 90 percent of thenoncondensible gases were composed of hydrogen sulfide. Consequently,the gases could be passed without further purification directly to theClaus oxidizer, and this is what was done.

The pressure of the recovered hydrogen sulfide mixture was increased to1.4 atmospheres in a piston compressor,

8 and the gas was then mixed with 700 normal cubic meters per hour ofair, preheated to 365 C., and was then reacted in a Claus reactor packedwith bauxite. The resulting sulfur, 18 kg. per ton of pulp, wascondensed at C., and passed to the polysulfide preparation system.

The combustion of the black liquor after evaporation was carried out sothat 7 kg. of sulfur per ton of pulp left with the combustion gases assulfur dioxide, which was not recovered. The requirement of make-upsulfur was 17 kg. sulfur per ton of pulp. This was added as elementalsulfur, and together with the recovered sulfur, afford 35 kg. ofpolysulfide per ton of pulp. The regenerated white liquor was then readyfor use for digestion of a further batch of wood.

Having regard to the foregoing disclosure, the following is claimed asthe inventive and patentable embodiments thereof:

1. A process for the economical recovery of hydrogen sulfide from thewaste gases obtained during the evaporation of alkaline black liquor,and particularly polysulfidecontaining alkaline black liquor, from thekraft pulping process and using the sulfur from the hydrogen sulfide toform a sulfide cooking liquor, comprising preparing the black liquor forevaporation in a manner which ensures the liberation of a relativelyhigh concentration of hydrogen sulfide in the effluent noncondensiblegases (compared to that normally liberated) and a high ratio of hydrogensulfide to oxygen in such gases, while at the same time holding to aminimum the volume of combustion gases brought in contact with theevaporating liquor, by carbonating the black liquor with combustiongases containing at least about 15 percent carbon dioxide, in an amountto reduce the alkaline black liquor to a pH below 11, the combustiongases being less than about 20 normal cubic meters per cubic meter ofunevaporated black liquor, and then stripping hydrogen sulfide from theresulting liquor, while evaporating the liquor to increase the solidscontent to at least 45 percent, said evaporation carried out withoutaddition of combustion gases, thereby removing a portion of the sulfurof the liquor as hydrogen sulfide, and forming a proportion of at least80 percent hydrogen sulfide in the noncondensible gases liberated fromthe black liquor, recovering hydrogen sulfide from the condensible gasesevolved in the evaporation, oxidizing the recovered hydrogen sulfide tosulfur with air or sulfur dioxide, and dissolving the sulfur in asulfide-containing aqueous solution to form a sulfide cooking liquor.

2. A process according to claim 1 in which the black liquor is apolysulfide black liquor and the sulfur is dissolved to form apolysulfide cooking liquor.

3. A process according to claim 1, in which the sulfidecontainingaqueous solution is obtained by combustion of the evaporated blackliquor, to form a residue comprising alkali carbonate and alkalisulfide, causticizing the residue to form alkali hydroxide and alkalisulfide, and dissolving the residue in Water.

4. A process according to claim 1 in which the carbon dioxide content ofthe combustion gases is within the range from about 15 to about 21percent.

5. A process according to claim 1, in which the quantity of availablehydrogen sulfide is less than 22 kg. per ton of pulp, and the amount ofcombustion gases is less than normal cubic meters per ton of pulp.

6. A process according to claim 1 in which the pH of the black liquor isreduced to within the range from about 11 to about 9.

7. A process according to claim 6 in which the amount of hydrogensulfide liberated during the stripping operation, based on the totalamount of sulfur in the liquor, is controlled within the range fromabout 9 to about 25 percent.

8. A process according to claim 1 in which the combustion gases are thegases that are obtained during the combustion stage of the evaporatedblack liquor.

9. A process according to claim 1 in which the evapora- 9 tion iseifected under vacuum and hydrogen sulfide is fed under pressure to theoxidizing step.

10. A process according to claim 1 in which the hydrogen sulfide isseparated from the gases evolved during the evaporation by absorption ina suitable absorbent, desorbed, and then fed to the oxidizing step.

7/1960 Kibrick 162-82 X 10 10 3,293,113 12/ 1966 Venernark 162-303,331,732 7/1967 Venemark 16230 3,331,733 7/1967 Venemark 162-30 I. LEONBASHORE, Primary Examiner R. H. ANDERSON, Assistant Examiner US. Cl.X.R. 23--48

